Method and automated system for control of oil well production and modular skid for use in said method

ABSTRACT

Automated measurement and oil well production control may be achieved by using a vertical separator, the discharge flow of which is continuously adjusted by setting the opening of a control valve, determined by the liquid level inside the separator. The automation of the control method allows real-time measurements of several process variables as well as reduced measurement times and also works as a safety layer for a production process. The control method is independent of well production and is therefore suited to controlling marginal wells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Application No. 61/710,507 filed on Oct. 5, 2012 under 35 U.S.C. §119(e), the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to an automated well production measurement and control system also working as a safety layer for the production process and to a modular skid for its implementation.

BACKGROUND OF THE INVENTION

Oil production is the process by which a reservoir fluid is transported to the surface to be separated into oil, gas and water. If necessary, the obtained oil and gas will be treated and conditioned for sale or transport from the field to a petroleum refinery. The so called “upstream operations” consist of the exploration, development and production of crude oil, water and natural gas.

The production process is carried out in a surface installation. This installation includes wells, manifolds, pipelines, production lines, separators and other process equipment, as well as measuring equipment and storage tanks.

During operation of an oil production process, well control is essential, as it is closely related to reservoir productivity and can be used for optimization of the surface installation. Measuring gross production and water cut is essential in determining the quality of the product for the next steps of the process.

The different control philosophies in well control require appropriate measurements of the variables of interest. In particular, the measurement of the flow produced by a well determines the control strategy. Therefore, there is an ongoing need to improve measuring methods in order to evaluate well production, particularly when dealing with marginal oil wells due to the greater content of water in the produced fluid.

In some traditional well control techniques, the measurement of average production rate is achieved by using a storage tank, hereinafter designated as “control tank”.

However, wells in marginal fields generally have production rates with a wide variability and a high water cut, by which in some cases, filling the control tank can be technically challenging, either with the volume produced over a 24 hour or an even longer time period in the case of a low production rate, or with the short time required to fill the tank in the case of a very high production rate.

For this reason, the procedure for well control using a control tank requires extrapolation of a partial measurement. This partial measurement is performed by using two timers, if included in the tank, or by registering the time elapsed between the start of the control process and the level measuring right before the end of the control process. Extrapolating this partial measurement to 24 hours can be a weak approximation, since it must be assumed that the well is in a stable production regime beyond the measured time lapse. In case no level sensors are available in the tank, the measurement is dependent on a manual time measurement, which further decreases the validity of the extrapolation procedure.

Yet another source of error is the measuring of the liquid level using a measuring tape. This is due not only to the error associated with a manual measuring, but also to the fact that the measuring action must be performed with a moving fluid inside the control tank, a situation explicitly counter-indicated in the API MPMS Standards.

In addition, there is a further source of error derived from the control tank calibration chart and its use, given that a manual interpolation must often be performed.

Finally, a single tank measurement is not enough to determine gross production. Technically, in order to terminate the control process, the observed temperature must be measured in order to determine the volume of the fluid contained inside the tank. Production temperatures of approximately 30 to 40° C. imply a high oil thermal expansion coefficient, which can result in a volumetric error greater than 1%.

In other well control systems, such as the one disclosed in U.S. Pat. No. 3,765,442 A, the measurement of average production rate is achieved by using an oil and gas separator, a float switch mechanism and a storage tank. The system may also be provided with an automatic counting system to count the total barrels produced by the entire group of producing wells during a particular day to thereby meet allowable limits for the field, if any.

While the main advantage of said system relies on the automation of the control procedure, there is limited capability for real-time measurement and monitoring of real-time variables, such as instantaneous and average well production, water cut, fluid density, temperature, etc. A further disadvantage of said system is the use of a storage tank.

The use of a barrel counter requires frequent maintenance procedures, as an adequate system calibration is required to ensure discharging a preset volume value during each tank emptying step.

There is thus a need to introduce a method for enhanced measurement of well production, with reduced errors compared to traditional measurement systems and allowing real-time measurements. Furthermore, it is desirable that the measurement does not depend on well production. Finally, there is also a need to reduce well testing times as well as the overall size of surface installations.

BRIEF DESCRIPTION OF THE INVENTION

A continuous measurement of the gross production and water cut is needed in order to optimize the performance of a surface installation for oil collection, also known as “battery”. This can be achieved by building a reliable production allocation system, which provides improved measurement accuracy and a better understanding of oil field behavior.

It is therefore an object of the present invention to provide a method for measurement and on-line control of the flow of a well produced fluid, also working as a safety layer for the production process, said method comprising:

-   -   providing a two-phase vertical separator for separating gas and         liquid streams from a stream of produced well fluid;     -   measuring the flow of the liquid stream by using a single flow         meter located downstream from the two-phase vertical separator;     -   simultaneously, measuring the level of liquid within the         two-phase vertical separator;     -   continuously adjusting the liquid discharge flow of the         two-phase vertical separator by setting the opening of its         control valve, located downstream from the two-phase vertical         separator and from the flow meter, so as the liquid level in the         two-phase vertical separator falls within a range of about 20%         to about 80% of its maximum operational level, in successive         cycles of filling and emptying of the two-phase vertical         separator.

The setting of the operational liquid levels may be made by the field operator according to the process conditions.

This method allows the flow meter to continuously work in its optimal range, where the measurement errors associated with the flow are minimized.

Besides, the method of the present invention allows a much faster control of well production, with a remarkable reduction in the time spent in control process when compared to the traditional control tank system.

Said time reduction is achieved as long as the wells show relatively steady gross production and water cut. On these conditions, the field operator may take the decision of ending the well control. This is not possible when applying the traditional, control tank system.

In preferred embodiments of the present invention, the flow meter is one selected from the group consisting of a Coriolis mass flow meter, a differential-pressure meter, an orifice meter, a positive-displacement meter, a vortex meter and a multiphase meter.

In a preferred embodiment of the method of the present invention, the flow meter is a mass flow meter.

In another preferred embodiment of the method of the present invention, the flow meter is a Coriolis type mass flow meter.

Additionally, in a preferred embodiment of the method of the present invention, the water cut of the produced fluid stream is measured by means of at least one water cut meter.

In a yet another preferred embodiment of the method of the present invention, the water cut of the produced fluid stream is estimated by means of a density measurement.

It is another object of the present invention to provide a measuring and monitoring portable modular skid comprising instruments for measuring and monitoring the flow properties of the produced fluid.

The modular skid of the present invention comprises:

-   -   a two-phase vertical separator with level sensor means and their         associated control system;     -   a flow meter located in the liquid discharge pipe of said         separator;     -   at least one water cut meter or a density measurement device;     -   a control valve located in the liquid discharge downstream from         the flow meter; and     -   optionally, a data processing system.

In a preferred embodiment of the modular skid of the present invention, the flow meter is a mass flow meter.

In preferred embodiments of the modular skid of the present invention, the flow meter is one selected from the group consisting of a Coriolis mass flow meter, a differential-pressure meter, an orifice meter, a positive-displacement meter, a vortex meter and a multiphase meter.

In another preferred embodiment of the modular skid of the present invention, the flow meter is a Coriolis type mass flow meter.

In another preferred embodiment of the modular skid of the present invention, the valve is a control valve with Micro-Form™ valve plug.

In a preferred embodiment of the modular skid of the present invention, the data processing system is part of a general industrial control system.

In a most preferred embodiment of the modular skid of the present invention, the industrial control system is based on a “Supervisory Control and Data Acquisition” (SCADA) system.

The implementation of the method of the present invention by incorporating the measurement skid to a manifold allows real-time measurement of well production with reduced error and reduced measurement times. Additionally, the method ensures that the measurement does not depend on the production of the wells and facilitates the automation of process control. It is thus possible to achieve a better performance on gross production and water cut measurement, as well as optimizing oil production due to the advanced diagnostics and flexible system configuration.

It is another object of the present invention to provide a measurement system that does not depend on well production, allowing real-time measurements, reduced control test times and reduced operating expenditures (OPEX) and capital expenditures (CAPEX).

Still another object of the present invention is to provide a safety system associated to the process, to ensure a safe process to the field operators and to avoid environment damage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart of a traditional production process layout, comprising a plurality of traditional batteries (B1, B2 . . . Bn) for oil collection each as shown in FIG. 2 a.

FIG. 2 a shows a flowchart of a traditional battery in an oil production process.

FIG. 2 b shows a flowchart of a battery with an automated control line according to the present invention, in an oil production process.

FIG. 3 shows a schematic view of a control tank.

FIG. 4 shows a flowchart of the method of the present invention according to a preferred embodiment.

FIG. 5 a shows trends obtained in field experiments for the liquid level transmitter position in the two-phase control separator of the embodiment of FIG. 4.

FIG. 5 b shows an example of the calculated average flow rate of a controlled well using the control method of the present invention in the embodiment of FIG. 4.

FIG. 6 shows field experiment results of the on-line measured water cut of a controlled well using the control method of the present invention according to the embodiment of FIG. 4.

FIG. 7 a shows a trend for the average flow of a well with a rod pump system, controlled by the control method of the present invention according to the embodiment of FIG. 4.

FIG. 7 b shows another trend for the average flow of a well with a rod pump system, controlled by the control method of the present invention according to the embodiment of FIG. 4.

FIG. 8 shows field experiment results for average gross production and instantaneous discharge flow rate for a well controlled with the control method of the present invention according to the embodiment of FIG. 4.

FIG. 9 a shows a trend of the measured instantaneous flow rate of a well with intermittent production by the control method of the present invention according to the embodiment of FIG. 4.

FIG. 9 b shows a trend of the measured instantaneous flow rate of a well stabilized by the control method of the present invention according to the embodiment of FIG. 4.

FIG. 10 a shows the position of the control valve in response to the safety system action and the calculated average production for a well in control, due to a change in process conditions.

FIG. 10 b shows the position of the control valve and the liquid level transmitter inside the separator, as a fraction of the maximum operating level, in response to the safety system action, due to a change in process conditions.

FIG. 10 c shows the position of the control valve and the separator discharge flow rate, in response to the safety system action, due to a change in process conditions.

FIG. 10 d shows the measured average temperature and pressure inside the separator, due to a change in process conditions.

FIG. 11 a shows a graph representing the relationship between the flow rate and the accuracy of the flow meter.

FIG. 11 b shows a graph representing the relationship between the flow rate and the accuracy of the flow meter.

FIG. 12 shows the trend of the average pressure and temperature inside the control separator measured by the control method of the present invention according to the embodiment of FIG. 4.

FIG. 13 shows a screen capture from the SCADA control system.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in detail below with reference to the figures.

FIG. 2 a shows a flowchart of a traditional battery in an oil production process. A manifold 1 is used for guiding a stream of produced fluid from a well, by means of one or more of various pump systems, like rod pumps, electrical submersible pumps (ESP), etc. Said manifold is connected to a control line 3, comprising a heater 2, a two-phase control separator 4, a control valve 8 and a control tank 9. The general installation line 5 comprises a two-phase general separator 6, a general tank 7, connected through a plurality of pumps 11 to a line leading to a treatment plant. The gross liquid production stored in the general tank 7 is pumped to the treatment plant by means of said pump assembly 11. The liquid is measured by a measuring bridge consisting of a mass flow meter and a water cut analyzer. Both the general separator 6 and the control separator 4 have an output for a gas line 10 which is sent to a compression plant.

FIG. 2 b shows a flowchart of a battery in an oil production process with an automated control line according to the present invention. A manifold 1 is used for guiding a stream of produced fluid from a well, by means of one or more of various pump systems, like rod pumps, electrical submersible pumps (ESP), etc. Said manifold is connected to a control line 3, comprising a heater 2, a two-phase control separator 4, a flow meter 13 and a control valve 8. The general installation line 5 comprises a two-phase general separator 6, a general tank 7, connected through a plurality of pumps 11 to a line leading to a treatment plant. The gross liquid production stored in the general tank 7 is pumped to the treatment plant by means of pump assembly 11. The liquid is measured by a measuring bridge 12 consisting of a mass flow meter and a water cut analyzer. Both the general separator 6 and the control separator 4 have an output for a gas line 10 which is sent to a compression plant.

FIG. 3 shows a schematic view of a control tank. The liquid level inside the tank is monitored by an external level indicator 21. The tank contains a siphon leg 22 which prevents overflow of the liquid inside the tank.

Once well control is ended, a sample is manually taken by the field operator using a fixed sampler 23. The sample is representative of the liquid profile inside the tank. The fluid inside the control tank is drained out through line 24, wherein the displaced fluid, corresponding to the well in control, is discharged to the general tank 7 as shown in FIG. 2 a.

With reference to FIG. 4, the liquid leaving the control separator 4 is guided through line 3 containing a flow meter that includes a water cut analyzer 13. A control valve 8 is located in the same line along with an actuator associated with a PLC system, which receives a signal from an electronic level measurement transmitter 14.

In order to account for the varying flow rates due to high production variability, which alters the accuracy in flow measurement by the flow meter at low flow rates, see FIG. 11 a, the chosen control system is a snap-action (ON-OFF) control system.

Making reference again to FIG. 4, the aim of this control is to set the discharge flow of the control separator 4 to a value determined by the opening of the control valve 8, controlled by the signal sent by the electronic level measurement transmitter 14 located in the control separator 4, allowing the flow meter to work within its optimal range of operation, thereby reducing the error associated with flow rate measurement, as shown in FIG. 11 b.

The operation of the measurement and control method comprises a series of cycles of filling and discharge of the control separator 4. Making reference to FIG. 4, said separator 4 is fed with a produced fluid stream from the production well to be controlled.

The separator 4 should be kept filled up to a certain required level, in order to ensure that the flow through the flow meter is within its operating range. Prior to the control of a well, this level data must be fed to the control system, in order to correctly set the various control variables.

During the filling step of the operating cycle, when the liquid level controller 14 indicates a 20% displacement of the sensor, i.e. 20% of the maximum operation level, the output control signal of the PLC electronic positioner acts on the control valve 8 and sets it in the closed position. The valve remains in this position until the liquid level transmitter 14 in separator 4 reaches 80% of the maximum operational level. Thus, the two-phase separator 4 is filled to the required level before discharge.

During the discharge step of the operating cycle, once the fluid level inside separator 4 is such that the level sensor 14 reaches 80% of the maximum operational value, the signal from the level controller is sent to the PLC, which acts upon the electronic positioner of the control valve 8, by sending a signal proportional to the displacement required to open the control valve.

The opening degree of the control valve 8 is therefore determined solely by the gross production of the controlled well.

During the discharge step of two-phase separator 4, the flow indicated by the flow meter 13 should be verified to be suitable to the well in control. Such flow rate value is determined by considering the flow range in which measurement errors of the flow meter are minimal. This value should be introduced in the general system settings.

When the liquid level in two-phase separator 4 causes the level to reach 20% of its maximum operational value, the filling step of the cycle is repeated, i.e. the control valve 8 is completely closed again, allowing filling of the two-phase separator 4.

Maximum operational level values are approximate and can be customized and set to meet the requirements of each particular installation.

By means of the on-off type control, the measurement is rendered independent of the flow rates of the oil production process, allowing selecting a unique flow meter to measure the full range of well productions arriving to a specific battery. Based on this, the measurement equipment can be designed and specified to the requirements set by the operator.

The system of. FIG. 4 comprising the level transmitter 14, the flow meter 13, the flow control valve 8 and the associated logic is the basis for the well control operation. As previously described, the main function of the level control 14 is operating on the opening and closing of the control valve 8. This, together with its primary function, provides the possibility of determining trends -for example well performance—over time.

Additionally, level transmitter 14 together with pressure transmitter 17, high level switch 16, low level switch 15, pressure control valve 20 and flow control valve 8 are also part of a safety layer of the production process.

With reference to FIG. 4, if the level of liquid inside separator 4 exceeds the range of operation of level transmitter 14, e.g. when the liquid level in the separator 4 reaches 90% of the maximum operational level, an alarm is activated, for example from a SCADA control system, allowing the field operator to take corrective actions if possible.

In case the level of liquid inside separator 4 reaches the high level switch 16, a signal issuing therefrom will act directly on control valve 8, causing a total opening of the valve. This action allows emptying the separator until the level transmitter 14 reaches 20% of its maximum operational value. This cycle is repeated until normal operation conditions are re-established. If this abnormal situation persists, the field operator may finalize well control, for example by means of a switch in a SCADA control system.

If the pressure inside separator 4 suddenly exceeds the normal operating pressure, a signal from pressure transmitter 17 will act on pressure control valve 20 and on control valve 8 in order to prevent the opening of pressure safety valve 18 and thus reduce the pressure value to a normal operating level.

Measurement automation may be achieved by the characteristics of the logic associated to the measuring devices, by use of an industrial control system such as SCADA control system, as shown in FIG. 13.

In well control, the use of measuring devices with associated control logic allows for monitoring instantaneous accumulated volumes, daily accumulated volumes, average flow of the controlled wells as shown in FIG. 5 b, and average water cut as shown in FIG. 6, among other variables. These variables can be calculated from the measurements by programming a PLC type controller.

The accumulated volume, based on the control system of the on-off type, is measured during each control separator discharge, and can be used to calculate the average well gross daily flow rate as shown in FIG. 8.

Additionally, the water cut measurement is performed during the period of time during which the control valve is open, as shown in FIG. 6.

By using a suitable display system, this information can be displayed in real time in order to monitor the behavior of the controlled well. Thereby, it is possible to track the instantaneous well production and its behavior over time, as shown in FIG. 9 a and FIG. 9 b. FIG. 9 a shows the results of the control of a well with intermittent production. This anomaly can be rapidly detected by the control method of the present invention. FIG. 9 b shows the production of a well with stable behavior during its control.

Since this information can be obtained in real time, this control method presents an advantage for the operator, as it allows earlier decisions regarding conventional control with longer measurement times.

Therefore, the measurement and control method which is an object of the present invention significantly allows reducing the well control times, and the size of surface installations (FIG. 2 b) and allows the taking of corrective actions in real time.

FIG. 7 a and FIG. 7 b show the trends for the average flow of a controlled well with a rod pump system using the control method of the present invention. FIG. 7 a corresponds to a well control process with a duration of about 10 hours and FIG. 7 b corresponds to the same control, but limiting the control time to approximately 6 hours. It can be seen that the method of control of the present invention allows well testing in real time, so that a decrease in control times is possible, as it was previously stated.

In addition, the decision to end a control process with the method of the present invention can be easily made by the field operator due to the real-time nature of the measurements.

The amount and quality of the data that can be gathered allows the collection of historical cumulative production of wells and the construction of correlation curves for studying the behavior of the wells over time.

The measurement and control method of the present invention also allows the reduction of the investment and operating costs associated with the systems comprising a control tank used in the traditional processes for well control.

Safety System Operation

FIGS. 10 a, 10 b, 10 c and 10 d show the field experiment response of the safety system associated to the use of the control method of the present invention.

In FIG. 10 a, the control valve was set to operate at 35% of its maximum opening-valve position trend shown by reference mark ▪. The average flow rate of the well in control is represented by reference mark ▴ and can be seen to have a tendency towards the value of 22 m³/day.

At a given time during well control, the average flow rate trend line shows a sharp increase in production, which precludes draining out the fluid inside the separator, with the normal operating conditions as set before starting well control. This condition causes “flooding” of the control separator.

In this undesirable condition, making reference to FIG. 10 b, the level transmitter shown by reference mark:  reaches a 100% displacement value, i.e. a value greater than the limit of 80% set before starting well control. This situation sets off an alarm, for example in the SCADA system (FIG. 13).

Due to the high rate of change in process conditions, precluding the taking of corrective actions by the field operator, the liquid level inside the control separator reaches the high level switch, sending a signal to the control valve and setting it to open to 100% of its maximum opening (“full open”). The curve for the control valve is represented with reference mark ▪.

The action of the control valve generates a relief in the control separator by means of the instantaneous evacuation of the liquid contained in it. This can be appreciated in FIG. 10 c, where the control separator discharge flow rate shown by reference mark ♦ increases from a value of approximately 110 m³/day in normal operating conditions to a value of 405 m³/day triggering the safety process operation.

FIG. 10 d shows the change in process conditions for the field experiment. The pressure inside the control separator is represented by reference mark ▾. It can be observed that in normal operating conditions, the measuring system works with a pressure of approximately 3.6 kg/cm². After the change in process conditions, a pressure increase inside the control separator is observed, reaching a value of approximately 4.5 kg/cm², triggering the safety process operation.

The control method of the present invention therefore provides a safety system which ensures a safe process to the field operators and avoids environment damage.

Monitored Variables

The present invention allows monitoring a series of variables which are essential to the well control operation. The relevant variables are described in detail below. It should be noted that the system may be further configured to also monitor secondary variables.

Average gross production of the controlled well and instantaneous discharge flow rate

By using the control system of the present invention, two flow rate data are obtained: instantaneous discharge flow rate, measured by the flow meter in each discharge step of the two-phase control separator, and average well flow rate (average well production), which is averaged over each discharge step of the two-phase separator.

It should be noted that the discharge flow rate of the control separator, set by the opening of the control valve, is not the actual gross well production.

Experimental results for average gross well production and instantaneous discharge flow rate are shown in FIG. 8, where the trend line of reference mark ▪ corresponds to the opening and closing of the control valve operating in ON-OFF mode. The trend line marked with reference mark ▴ corresponds to the average flow rate of the controlled well. The value of this variable can be loaded as an input to the industrial control system of the company. The trend line marked with reference mark ♦ is the instantaneous discharge flow rate measured by the flow meter in each discharge step of the two-phase control separator. This flow rate is determined by the opening of the control valve.

Well Behavior—Instantaneous Production

The control method of the present invention allows visualization of the instantaneous flow rate entering the two-phase control separator for the controlled well.

This variable can be used to adjust settings of the pumping regimes, in case of deviations from the optimal operation behavior.

FIG. 9 a shows the trend of the measured instantaneous flow rate corresponding to an inadequate well behavior or pumping system operation, resulting in an intermittent production of the controlled well. FIG. 9 b shows the trend of the measured instantaneous flow rate for a stabilized well, by means of the method and skid of the present invention.

Average Water Cut

The control method of the present invention allows visualization of the average water cut of the controlled well.

A field experiment result for average water cut well production is shown in FIG. 6. This variable is averaged over each discharge step of the two-phase separator.

Average Process Temperature and Pressure

The control method of the present invention allows visualization of the average pressure and temperature trending inside the two-phase control separator.

An experimental result for both average pressure and temperature are shown in FIG. 12. The average pressure curve is represented by reference mark +. The average temperature curve is represented by reference mark ♦.

Others monitored process variables may include: liquid level inside the two-phase separator, instant and/or average fluid density, instant and/or average flow meter temperature, communication status, test time, alarms, etc.

In view of the above, implementing the control method and skid of the present invention results in several benefits in a well control process, including:

-   -   increase in measurement quality by reduction of measurement         error: the characteristics of the on-line well control and         monitoring method of the present invention provide a superior         data quality and therefore an improvement in the dynamic model         of a reservoir for resource estimation;     -   continuous production monitoring (well in control): the system         allows visualization of real-time production trends for the         controlled well. This information can be used for the real-time         action-taking for correcting deviations of well behavior from         the optimal operation regime;     -   reduction of well control times: the method of the present         invention allows increasing the frequency of well controls due         to the shortening of the control times compared to traditional         well control systems. This increased frequency is also         translated into a larger data volume, allowing obtaining         historical trends and correlation curves to study well behavior;     -   obtaining historical well data: the method of the present         invention (automated well control system) allows gathering         information related to well controls, which can be used to         obtain real data for historical well behavior;     -   cost reduction: the method of the present invention provides         better results regarding gross production and water cut         measurements compared to traditional well control while using         less equipment (control tank), resulting in a reduction of the         capital and operational expenditures of a surface installation;     -   measuring water cut in an extended range (0-100%) with known         measurement error: the use of on-line water-cut measurement         equipment allows defining a range of measurement error, which is         integrated to the final error of the measurement system.     -   real-time monitoring of variables: by means of a SCADA software,         the monitoring of well control in real time is possible,         allowing the user/operator to begin and terminate well control         from a control room, as well as monitoring variables and         adjusting settings in real time;     -   increase in production field operator knowledge: the         implementation of an automated control system, with the         possibility of a real-time measurement of variables, allows the         field operator to learn and understand the behavior of a well in         detail, due to the on-line nature of the available data;     -   implementation of a safety system for the process: by means of         process and control automation, the operator and the technical         integrity of the surface installation and the surrounding         environment can be protected from unsafe process events. The         on-line and real-time nature of the data acquisition system         allows the possibility to predict undesired events as well as to         take early warnings and/or preventive actions. 

1. A method for measurement and on-line control of the flow of a well produced fluid, also working as a safety layer for the production process, said method comprising: a. providing a two-phase vertical separator for separating gas and liquid streams from a stream of produced well fluid; b. measuring the flow of the liquid stream by using a single flow meter located downstream from a two-phase vertical separator; c. simultaneously, measuring the level of liquid within the two-phase vertical separator; d. continuously adjusting the liquid discharge flow of the two-phase vertical separator by setting the opening of its control valve, located downstream from the two-phase vertical separator and from the flow meter, so as the liquid level in the two-phase vertical separator falls within a range of about 20% to about 80% of its maximum operational level, in successive cycles of filling and emptying of the two-phase vertical separator.
 2. Method according to claim 1, wherein said range is of 20% to 80%.
 3. Method according to claim 1, wherein the flow meter is one selected from the group consisting of a Coriolis mass flow meter, a differential-pressure meter, an orifice meter, a positive-displacement meter, a vortex meter and a multiphase meter.
 4. Method according to claim 3, wherein the flow meter is a Coriolis mass flow meter.
 5. Method according to claim 1, wherein the valve is a control valve with Micro-Form™ valve plug.
 6. A modular skid for measurement and control of the oil well production, comprising: a. a two-phase vertical separator with level sensor means and their associated control system; b. a flow meter located in the liquid discharge pipe of said separator; c. at least one water cut meter or a density measurement device; d. a control valve located in the liquid discharge downstream from the flow meter; and e. optionally, a data processing system.
 7. A modular skid according to claim 6, wherein the flow meter is one selected from the group consisting of a Coriolis mass flow meter, a differential-pressure meter, an orifice meter, a positive-displacement meter, a vortex meter and a multiphase meter.
 8. A modular skid according to claim 7, wherein the flow meter is a Coriolis mass flow meter.
 9. A modular skid according to claim 6, wherein the valve is a control valve with Micro-Form™ valve plug.
 10. A modular skid according to claim 6, wherein the data processing system is part of an industrial control system.
 11. A modular skid according to claim 7, wherein the industrial control system is based on a “Supervisory Control and Data Acquisition” (SCADA) system. 